Conventional RF amplifiers required to simultaneously amplify RF signals that have large peak-to-average ratios, are costly and relatively inefficient (i.e., consume much DC power). However, during most of the time, the power output is only a small fraction of the power consumed from the Direct Current (DC) power supply, resulting in low efficiency. The reason for such inefficiency is that on one hand, the DC conditions should be set to values that will be able to provide large power output, but on the other hand, for these DC conditions, such a power amplifier becomes efficient only during the occurrence of the peaks, i.e., when the instantaneous power output is large and the power amplifier is at saturation level.
In many wireless applications, controlling output power as a function of receiving level is required. Using back off from saturation level will reduce efficiency. Conventional power stages are designed to handle the highest power, leading to lower efficiency at lower output power.
A conventional technique for eliminating this problem is to control the DC supply of the power amplifier as a function of power level. One voltage level is supplied to such power amplifier whenever the transmitted power is below a given level (normal operating condition), and an enhanced (and higher) voltage level whenever the output power is above said level. The technique of eXcess eNvelope eNhancement (XNN®) for power amplifiers (PAs), particular for the low power WiFi and WiMAX market applications is disclosed in U.S. Pat. No. 6,437,641. This technique is a simplification of Envelope Tracking (ET), which is disclosed in US 2004/0018821.
The Voltage Enhancement Circuit (VEC™), disclosed in U.S. Pat. No. 6,831,519, modulates the supply voltage of power amplifiers as part of the XNN® technique. Appropriate modulation of the supply voltage prevents saturation of the power amplifier, while amplifying signals that exceed a pre-defined (fixed) programmable threshold. The resulting input drive to the PA is increased, thereby pushing its output well into saturation and eliminates the problem of signal clipping by instantaneously enhancing the power supply voltage.
The methods described above provided solutions only to the problem of improving the efficiency of power amplifiers operated under large peak-to-average ratios, while eliminating the need for clipping signals having large peak amplitudes. The XNN® technology suggests dynamically boosting the drain voltage of the PA to enlarge the output dynamic range of the PA.
All the methods described above have not yet provided satisfactory solutions to the problem of automatically controlling the XNN® enhancement threshold level while operating at various input levels and in a changing environment, in which the supply voltage, the temperature and other varying parameters affect the RF level monitored by the XNN® circuit.
It is therefore an object of the present invention to provide a method and apparatus for automatically controlling the XNN® enhancement threshold level, while operating at various input levels.
It is another object of the present invention to provide a method and apparatus for automatically controlling the XNN® enhancement operation with power amplifiers to be adjusted for different power amplifiers and different input signals.
It is a further object of the present invention to provide a method and apparatus to improve the enhancement operation of XNN® circuits/chips (integrated circuits) compared to XNN® chips with fixed threshold level.
It is yet another object of the present invention to provide a method and apparatus to compensate the enhancement operation of XNN® chips against gain variations in the power amplifier and against variations between XNN® chips during manufacturing.
Other objects and advantages of the invention will become apparent as the description proceeds.